(a) Field of the Invention
The present invention relates to a distributed feedback (DFB) semiconductor laser device and more particularly, to a DFB semiconductor laser device including a diffraction grating which can be formed with excellent controllability and excellent reproducibility as well as in-plane uniformity over an entire wafer surface. The present invention also relates to a method for fabricating such a DFB semiconductor laser device.
(b) Description of the Related Art
A DFB semiconductor laser device (referred to as simply DFB laser or DFB laser device, hereinafter) has a selection property for the emission wavelength thereof by using a feedback loop for effecting a selective feedback of a specified wavelength. The selective feedback is achieved by a diffraction grating which provides the laser active layer with a periodic spatial structure, wherein either the real part or the imaginary part of the complex refractive index of the laser device changes periodically with the location. The DFB laser operates at an excellent single mode, can emit laser of a stable single wavelength, and thus is widely used as the light source in optical communication systems.
The diffraction grating of the DFB laser is disposed in the vicinity of the active layer of the DFB laser and includes a grating layer having a periodic refractive index which is different from the refractive index of the adjacent layers. The emission wavelength xcexDFB of the DFB laser is determined by the formula xcexDFB=2neffxcex9, wherein xcex9 is the spatial period of the diffraction grating and neff is the effective refractive index of the waveguide of the laser device. Thus, the emission wavelength xcexDFB can be determined independently of the peak wavelength of the optical gain of the active layer. The DFB laser is categorized into two types including a refractive-index coupling type and a gain coupling type based on the material of the diffraction grating.
A DFB laser of the refractive-index coupling type has a diffraction grating made of a transparent material which allows the emission wavelength to path therethrough, wherein the real part of the complex refractive index changes periodically with location in the laser device.
On the other hand, a DFB laser of the gain coupling type has a structure wherein the imaginary part of the complex refractive index changes periodically with location in the laser device. The gain coupling type is again categorized into three subtypes including a first subtype (or gain grating type) wherein the diffraction grating is formed in the active layer structure itself to obtain a periodic gain distribution with respect to location, a second subtype (or absorption grating type) wherein a material for absorbing the emission wavelength is periodically disposed in the vicinity of the active layer to obtain a periodic absorption distribution with respect to location, and a third subtype (or current blocking type) wherein a material for blocking the injection current is periodically disposed in the vicinity of the active layer to obtain a periodic carrier density distribution with respect to location.
The gain grating type has a higher gain-coupling coefficient, whereby it is expected that the gain grating type achieves a higher product yield in fabrication of a single mode laser device. In addition, since a DFB laser of the gain grating type generally operates with an in-phase mode at a longer wavelength of the Bragg""s wavelengths, it is also expected that the gain distribution has an advantage of maintaining a stable single mode even at a higher injection current.
The gain grating type DFB laser is expected for use as a light source in an optical communication system due to the advantage of higher coupling coefficients, which are important parameters of the DFB laser, as described above, especially due to the advantage of the higher gain coupling coefficient.
However, there is a problem in the reproducibility and the controllability of the gain coupling coefficient in the gain grating type DFB laser device, resulting from the poor controllability of etching during the etching of the active layer structure to form the diffraction grating therein. This problem causes a poor product yield in fabrication of the DFB laser devices having an accurate coupling coefficient as designed.
More specifically, since the diffraction grating is formed by etching the active layer structure in the gain grating type DFB laser, the etching accuracy in the active layer structure significantly affects the coupling coefficient and the laser characteristics, which may have larger dispersions. A smaller coupling coefficient degrades the wavelength selectivity, whereby a multi-mode lasing occurs in the DFB laser. On the other hand, a large coupling coefficient increases the probability for occurrence of a spatial-hole-burning in the longitudinal direction of the laser device, whereby the operation at a higher injection current becomes unstable.
In this respect, it is to be noted that the electric field of the light assumes a maximum in the vicinity of the active layer, which in general constitutes the core of the waveguide. Thus, the diffraction grating formed in the active layer structure wherein the electric field of the light assumes the maximum causes the problem that the poor etching accuracy in the active layer affects the accuracy of the coupling coefficient. Even a small deviation in the thickness of the diffraction grating due to the poor etching accuracy significantly changes the coupling coefficient.
In particular, in the case of an active layer scheme having a multiple quantum well (MQW) structure, it is difficult to accurately control the etching depth in the current etching technique, and thus it is difficult to form a diffraction grating having accurate dimensions based on the design thereof.
Recently, it is common to employ an active layer scheme having the MQW structure in order to improve the laser characteristics, the MQW structure being such that a plurality of quantum well layers each having a thickness around 5 nm and a plurality of barrier layers each having a thickness around 10 nm are alternately layered. When a diffraction grating is formed in the MQW active layer structure by etching thereof, it is difficult to accurately control the etching depth or the resultant height of the grooves in the diffraction grating. In particular, it is difficult to obtain an in-plane uniformity of the etching depth for the MQW active layer structures over the entire wafer surface. This degrades the product yield of the gain grating type DFB lasers.
It is therefore an object of the present invention to provide a gain-coupling DFB semiconductor laser device of the gain grating type, which has excellent in-plane uniformity over the wafer surface and can be fabricated with an excellent product yield.
The present inventors noted that the controllability, reproducibility and in-plane uniformity of the diffraction grating could be improved by incorporating a compound semiconductor layer within the active layer structure, the compound semiconductor layer acting as a quantum well layer or a barrier layer in the MQW active layer structure as well as acting as an etching stop layer for the etching of the MQW active layer structure. The present invention is based on this principle.
Thus, the present invention provides a DFB laser device including a compound semiconductor substrate, a multiple-quantum-well (MQW) active layer structure overlying the compound semiconductor substrate and including a plurality of quantum well (QW) layers and at least one barrier layer, and a diffraction grating including a grating layer structure and an embedded layer embedded in the grating layer structure, the grating layer structure including at least one of the QW layers and the barrier layer of the MQW active layer structure, the embedded layer having a composition different from a composition or compositions of the QW layers, the QW active layer structure including an etching stop layer underlying the diffraction grating in contact therewith.
The present invention also provides a method for fabricating a DFB laser device including the steps of: forming a MQW active layer structure overlying a compound semiconductor substrate and including a plurality of quantum well (QW) layers and at least one barrier layer, one of the QW layers and the at least one barrier layer constituting an etching stop layer; and etching a portion of the MQW active layer structure overlying the etching stop layer in contact therewith to form a diffraction grating in the portion of the MQW active layer structure, the etching stop layer having a lower etching rate compared to an etching rate or etching rates of the portion of the MQW active layer structure during the etching.
In accordance with the DFB laser of the present invention and the DFB laser device fabricated by the method of the present invention, the provision of the etching stop layer in the MQW active layer structure improves the in-plane uniformity of the etching depth in the active layer structure for forming the diffraction grating, whereby the in-plane uniformity of the laser characteristics and thus the product yield of the DFB laser devices of the gain grating type can be improved.